1. Field of the Invention
This invention relates to switches for interrupting and commutating very large DC currents and has particular application to switches used in switching the very large DC currents employed in the electromagnetic propulsion of projectiles.
2. Description of the Prior Art
When very large currents are interrupted or commutated into another circuit, massive arcs are created at the opening contacts. These arcs can do great damage and severely limit the useful life of switch contacts if not properly handled.
One application where very large DC currents must be switched is in electromagnetic propulsion of projectiles. In such an arrangement, a projectile is placed between two parallel, electrically conductive launcher rails. The electromagnetic forces generated by the injection into the launcher rails of a very large DC current, on the order of hundreds of thousands of amperes, drives the projectile down the rails and out of the muzzle at very high velocities, on the order of several thousand meters per second.
The very large DC current is injected into the launcher rails by a firing switch. Typically this firing switch has been a rail switch, which is actually a second parallel rail device with one of its rails connected to the breech end of each of the launcher rails with a nonconducting section inbetween. An armature, which is in sliding electrical contact with the switch rails, is driven down the switch rails by the electromagnetic forces generated by the very large DC current to be switched. When the armature passes the nonconducting section of the one switch rail, a massive arc is struck between the armature and the section of the one rail it is leaving. As the armature continues down the switch rails, the arc lengthens, thereby increasing arc voltage which results in the injection of the current into the launcher rails. When commutation of the current is completed, the arc is extinguished.
While the rail switch provides rapid commutation of the current into the launcher rails due to the high speed (on the order of 50 meters per second) of the switch armature at the time the arc is struck, it is bulky, expensive, and requires means for stopping the armature after commutation, for returning it to the starting point and for restraining it against the forces generated by the applied current preparatory to a second firing. Thus the rail switch is also not adequately suitable for burst, or rapid, firing of the rail launcher.
In one type of rail launcher arrangement, the very large DC current is provided by a kinetic energy device such as a homopolar generator which charges a large inductor. The inductor, which is connected in series with the homopolar generator and the firing switch, stores inductive energy and releases it into the launcher rails upon actuation of the firing switch. In such an arrangement, it is known to provide a power switch which isolates the homopolar generator while it is coming up to speed, then connects the generator to the inductor and firing switch for the firing sequence, and finally, when firing is completed, disconnects the generator while connecting the inductor across the firing switch to dissipate the charge remaining in the inductor. Heretofore, this power switch has also been a rail switch with the attendant shortcomings discussed above (see my commonly owned copending application Ser. No. 207,568 filed Nov. 17, 1980). In a somewhat similar arrangement, a clamping switch in the form of conductive fingers which are rectilinearly inserted between bus bars has been used to short-circuit the homopolar generator during firing. This arrangement requires powerful hydraulic actuators to overcome the large electromagnetic forces resisting insertion of the conducting fingers.
The primary objective of the invention is to provide a switch for switching very large DC currents which is compact, is suitable for burst firing and does not require restraint of an armature preparatory to firing.
It is also an object of the invention to provide such a switch which is resistant to arc damage.
Other objects will become evident from a reading of the following summary of the invention and the description of the preferred embodiments.